Natalie Bonneton

Turbulent vortex dipoles in a shallow water layer

This paper describes an experimental study on turbulent dipolar vortices in a shallow water layer. Dipoles are generated by an impulsive horizontal jet, by which a localized three-dimensional turbulent flow region is created. Dipole emergence is only controlled by the confinement number C=Q/H2tinj whereas the jet Reynolds number Re=Q/v has no influence in the studied range 50 000 2, the flow becomes quasi-two-dimensional and a single vortex dipole emerges in most cases. By qualitative observations and application of particle image velocimetry, the main dipole features have been determined. The shallow water dipoles are characterized by the simultaneous presence of several scales of turbulence: A quasi-two-dimensional main flow at large scale and three-dimensional turbulent motions at small scale. A vertical circulation takes place in the dipole front. A theoretical model i...

Tidal bore dynamics in funnel-shaped estuaries

The formation and dynamics of tidal bores in funnel-shaped estuaries is investigated from both a global tidal wave scaling analysis and new quantitative field observations. We show that tidal bore occurrence in convergent estuaries can be estimated from a dimensionless scaling parameter characterizing the relative intensity of nonlinear friction versus local inertia in the momentum equation. A detailed analysis of tidal bore formation and secondary wave structure is presented from a unique long-term database (observations of more than 200 tides) acquired during four campaigns in the two main French tidal-bore estuaries: the Seine and Gironde/Garonne estuaries. We describe the effect of freshwater discharge on the global tidal wave transformation at the estuarine scale and on local tidal bore occurrence in the upper estuary. Our field data suggest that the tidal bore intensity is mainly governed by the dimensionless tidal range, which characterizes the local tidal wave nonlinearity. We also show that the secondary wavefield associated with tidal bore propagating in natural estuaries differs significantly from those associated to undular bores in rectangular channels. In particular, we observe an abrupt decrease of the whelp steepness when the Froude number goes below 1.1. This secondary field transition can explain why tidal bore occurrence in worldwide estuaries is certainly underestimated.

Rip current system over strong alongshore non-uniformities: on the use of HADCP for model validation

Modeling and understanding topographically-controlled rip currents remains a challenging task. One of the reasons is the lack of intensive, high-spatial resolution, flow field measurements in the rip channel vicinity. During the ECORS (DGA-SHOM) intensive field measurements, an intertidal inner-bar rip channel was instrumented with fixed eulerian current meters. In addition, for the first time in such a system, a Horizontal ADCP (HADCP) was implemented in the vicinity of the rip current, on the sandbar edge, for horizontally profiling wave induced-currents. Results show that the HADCP provides unique information on the shear in the vicinity of the rip neck, which is particularly useful for model calibration. The HADCP data was compared with local flow measurements for various tide and wave conditions, showing a very good agreement at a 5 m range. Restrictions and recommendations for HADCP implementation in the field are pointed out. The use of HADCP for horizontally profiling rip current circulations would benefit from being deployed outside of the breakers to measure the cross section of the rip head where sediment plumes and bubbles are essentially surface dominated. In this rip current system area, which would suffer from acoustic opacity only during high energy conditions, the rip current jet is strongly unstable owing to the current shear. HADCP would provide unique information on the rip current instabilities and vortex shedding in this poorly understood area of the rip current system.

High Frequency Field Measurements of an Undular Bore Using a 2D LiDAR Scanner

The secondary wave field associated with undular tidal bores (known as whelps) has been barely studied in field conditions: the wave field can be strongly non-hydrostatic, and the turbidity is generally high. In situ measurements based on pressure or acoustic signals can therefore be limited or inadequate. The intermittent nature of this process in the field and the complications encountered in the downscaling to laboratory conditions also render its study difficult. Here, we present a new methodology based on LiDAR technology to provide high spatial and temporal resolution measurements of the free surface of an undular tidal bore. A wave-by-wave analysis is performed on the whelps, and comparisons between LiDAR, acoustic and pressure-derived measurements are used to quantify the non-hydrostatic nature of this phenomenon. A correction based on linear wave theory applied on individual wave properties improves the results from the pressure transducer (Root mean square error, R M S E of 0 . 19 m against 0 . 38 m); however, more robust data is obtained from an upwards-looking acoustic sensor despite high turbidity during the passage of the whelps ( R M S E of 0 . 05 m). Finally, the LiDAR scanner provides the unique possibility to study the wave geometry: the distribution of measured wave height, period, celerity, steepness and wavelength are presented. It is found that the highest wave from the whelps can be steeper than the bore front, explaining why breaking events are sometimes observed in the secondary wave field of undular tidal bores.

High rate GNSS measurements for detecting non-hydrostatic surface wave. Application to tidal borein the Garonne River

Abstract This study presents the results of the use of high frequency Global Navigation Satellites Systems (GNSS) data for the detection and the characterization of tidal bores. The experiment took place the 31st of August 2015 in the Garonne River, at 126 km upstream the mouth of the Gironde estuary. The GNSS data acquired on a buoy at a sampling rate of 20 Hz were processed with a differential GNSS technique using the RTKLIB freeware. GNSS-based elevation of the free surface provides accurate estimates of the tidal bore first wave amplitude (1.34 m) and period (3.0 s). These values were in good agreement with values obtained using ADCP measurements.

Field Observations of Wave-Induced Set-Up on the French Aquitanian Coast

We report and analyze extreme wave-induced set-up heights obtained during a large international field experiment on the Atlantic coast of France. The field experiment associated with a large storm with the maximum offshore wave height of more than 12 m, enabled us to record extreme set-up heights up to 2 m. Such extreme data which are necessary for developing further numerical and analytical studies in this field, were lacking in the literature. Our data agrees reasonably well with existing set-up data reported from other coasts in the world. A good correlation was observed between set-up and offshore wave height. Similar to other coasts, the setup-offshore wave height relationship was linear up to a value of about 1 m. Nonlinear behaviour was observed for higher setup values. This study will help to further improve and validate the existing analytical and numerical solutions.Copyright

Dynamics of large-scale vortices in the near shore

In spite of the ubiquity of long shore flows, rip currents and also tidal jets in the near shore, large-scale vortices associated with these phenomena remain poorly understood. In particular, little is known about the effect of the vertical confinement on vortex dynamics. To understand this phenomenon we present in this paper a new laboratory experiment on pulsed jets (assimilated to a rip current) in a shallow water layer. In this study we analyze the evolution of three-dimensional turbulence generated by a pulsed jet in a homogeneous shallow water layer. We show that the jet evolution depends meanly on one dimensionless parameter c = (√Q/H²) tinj (where Qtinj the injected momentum flux, H the water depth and the injection duration). C characterizes the vertical confinement. When C is weak, the jet spreading is free. The generated turbulence remains fully three-dimensional. When C is large (C>2), we observe a damping of the vertical motion and the formation of a large horizontal dipolar structure. We also identify inside the global 2D dipolar structure, local three dimensional vortices. We have developed a theoretical model in good agreement with these measurements.